The invention relates to nematic liquid crystal mixtures having a positive dielectric anisotropy and an optical birefringence of at least 0.18, said mixtures being useful for an active matrix liquid crystal display (AMD) being operated in the second or a higher transmission minimum of the Gooch-Tarry curve and to novel fluorinated chloroterphenyls.
Active matrix displays (AMD) are highly favored for commercially interesting displays with a high information content. Such AMDs are used for TV application (e.g. for projection systems) and also for displays for e.g. laptops, automobiles and airplanes.
AMDs have non-linear electrical switching elements which are integrated at each picture element. As non-linear driving elements thin film transistors (TFT) [Okubo, U., et al., 1982, SID 82 Digest, pp. 40-41] or diodes (e.g.: metal insulator metal: MIM) [Niwa, K., et al., 1984, SID 84, Digest, pp. 304-307] can be applied. These non-linear driving elements allow use of an electro-optical effect with a rather flat electro-optical characteristic if a good viewing angle characteristic can be obtained. So a TN-type LC cell [Schadt, M. and Helfrich, W., 1971, Appl. Phys. Lett., 18, 127] with a twist angle of 90.degree. can be used. To provide good contrast over a wide viewing angle, operation in the first minimum of transmission [Pohl, L., Eidenschink, R., Pino, F. del., and Weber, G., 1980, German Pat., DBP 30 22 818, and 1981, U.S. Pat. No. 4,398,803; Pohl, L. Weber, G., Eidenschink, R., Baur, G., and Fehrenbach, W., 1981, App1. Phys. Lett., 38, 497; Weber, G., Finkenzeller, U., Geelhaar, T., Plach, H. J., Rieger, B., and Pohl, L., 1988, Int. Syrup. on Liq. Cryst., Freiburg, Liq. Crys. 1989, Vol. 5 pp. 1381-1388] is required. These AMDs are very well suited for TV applications and consequently are of high commercial interest. For these applications some physical properties of the liquid crystals become more important than for passive TN displays. Some of the decisive properties for the performance of an AMD are resistivity and stability of the liquid crystal [Togashi, S., Sekiguchi, K., Tanabe, H., Yamamoto, E., Sorimachi, K., Kajima, E., Watanabe, H., Shimuzu, H., Proc. Eurodisplay 84, September, 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 144 ff, Paris; Stromer, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. A problem often encountered is the adverse influence of UV-illumination on the resistivity and therefore on the general performance of the liquid crystal mixture in the display.
In an AMD the non-linear switching elements are addressed in a multiplex scheme, and charge the electrodes of a pixel in the limited time they are active. Then they become inactive until they are addressed again in the next cycle. Consequently the change of the voltage on an activated (charged) pixel is a nondesired but a very decisive feature of such a display. The discharge of a pixel is determined by two factors. These are the capacity of the pixel element including liquid crystal and resistivity of the dielectric material between the electrodes, namely the liquid crystal. The characteristic time constant of the decay of the voltage at a pixel (RC-time) has to be significantly bigger than the time between two adressing cycles (t.sub.adr.). A parameter frequently used to describe the performance of an AMD is the voltage holding ratio HR of a picture element: ##EQU1## As the voltage at a pixel decays exponentially an increase of the holding ratio necessitates liquid crystal materials with exceptionally high resistivities.
There are several points of importance for the resistivity of the liquid crystal inside a display, e.g. orientation layers, curing condition of the orientation material. But by no means less important are the electrical properties of the liquid crystal used. Especially the resistivity of the liquid crystal in the display determines the magnitude of the voltage drop at the pixel.
Earlier investigations with low-.DELTA.n materials have shown, that the requirements with regard to resistivity and UV-stability and temperature dependence of the resistivity for TFT-applications cannot be met with materials containing cyano moieties as terminal groups. Non-cyano materials containing halogenated terminal groups can show for better resistivity values and UV-stability as well as superior viscosity values than conventionally used cyano materials. However, in general these non-cyano materials unfortunately show a strong tendency towards forming crystalline and/or smectic phases, especially at low temperatures. Also the clearing points and the dielectric anisotropy values of non-cyano materials with halogenated terminal groups are much lower.
Modern commercial mixtures have to operate over a wide temperature range; therefore, crystallization or formation of smectic phases at low temperatures has to be excluded. Good solubility is one of the most important preconditions for the usability of liquid crystalline materials in the development of nematic mixtures. Compounds with high melting temperatures or a tendency to form smectic phases are for this reason not suitable.
By very careful selection of the components and an appropriate mixture design it was possible to find low birefringence non-cyano mixtures having a broad nematic temperature range for first minimum application [B. Rieger et al., Proc. 18. Freiburger Arbeitstagung Flussigkristalle, Freiburg 1989, 16 (1989)]. Non-cyano materials with high birefringence, which are essential for the mixture concept of this invention unfortunately show in many cases even more unfavorable properties such as high melting points and/or strongly smectogenic behavior than similar materials with lower birefringence:
______________________________________ No. Chemical structure .DELTA.n Mesophases (.degree.C.) ______________________________________ ##STR3## 0.126 K 70 S 79 N 193 I 2 ##STR4## 0.199 K 142 N 192 I 3 ##STR5## n.m. K 105 S 245 I ______________________________________
Mixtures of the state of the art with a birefringence suited for operation in the second or a higher transmission minimum of the Gooch-Tarry curve are not acceptable for active matrix application.
There is thus still a great need for liquid-crystal composition having a high resistivity and other suitable material properties for use in AMDs.
The chloroterphenyls No. 3 as shown above are known from JP 60-056 932-A. As outlined above these compounds do not allow meeting the severe specifications from the electronic industry, especially in view of their limited solubility in other LC materials, their high melting points and their pronounced smectogenity. Accordingly there is also a need in the art for improved non-cyano high birefringence LC compounds.